If you still didn't know which version to get, maybe these exclusive 'mons will sway you one way or the other.
If you purchase Sun, you will be able to catch Passimian, a fighting type monkey with the unique ability Receiver. Receiver allows Passimian to take your other Pokémon's ability once that one faints in a double battle. Sun version also gets a unique evolution to Rockruff, the normal/rock dog Pokémon already announced. Beyond appearance, nothing appears to be different between the two evolutions for Rockruff, both named Lycanroc.
For Moon version, you get Oranguru, a nomal/psychic type Orangutan. Orangutan comes with an exclusive move, Instruct, which lets your second Pokémon use the same move twice in one turn, in a double battle. We also get a look at the "Midnight Form" of Lycanroc.
The trailer also shows off more customization options for trainers and a mode that lets you groom your Pokémon. To see all the new stuff, just check out the trailer below.
(Source: The Official Pokemon Channel)